Magnesium oxide (hereinafter, frequently referred to as “MgO”) has conventionally been used in a wide variety of fields, such as refractory, electronic materials and catalysts. For example, in the field of plasma display panel (hereinafter, frequently referred to as “PDP”) utilizing an electric discharge phenomenon, specifically, alternating current type (AC type) PDP having a structure in which a transparent electrode is covered with a glass dielectric, for preventing the surface of the dielectric layer from changing in properties due to sputtering by ion bombardment to increase the discharge voltage, a protective film is generally formed on the dielectric layer. In this protective film, an MgO film has conventionally been used. The protective film is required to have a low discharge voltage and excellent resistance to sputtering. By using an MgO film as the protective film, which is an insulator having excellent resistance to sputtering and a large coefficient of secondary electron emission, the discharge starting voltage can be lowered. Thus the MgO film contributes to the increase of the life of PDP.
Conventionally, a ground product of MgO single crystals has been mainly used in the MgO deposition material for PDP. As the performance of PDP is improved, the PDP manufacturers demand a higher level of technique, and further improvement of the deposition material for PDP is desired. For this reason, in the deposition material for PDP, a sintered product obtained by sintering high-purity polycrystalline MgO particles, which can be easily controlled in the amounts of the elements added, is being used instead of the ground product of MgO single crystals. Further, in the electronic materials for applications other than PDP, raw materials for fluorescent substance, catalyst materials, raw materials for various target materials, raw materials for superconducting thin film substrate, raw materials for tunnel harrier for tunnel magnetoresistive element (TMR element) and catalysts, there are increasing demands for high-purity MgO materials.
As a technique for obtaining high-purity magnesium oxide, for example, patent document 1 (Japanese Unexamined Patent Publication No. Sho 61-209911) has a description concerning a method for producing high-purity magnesium oxide. The method described in this patent document is a method for producing magnesium oxide, wherein the method comprises: dissolving a crude raw material containing magnesium in a mineral acid to obtain a crude solution of a mineral acid salt of magnesium; adding an alkali to the crude solution to cause precipitation of impurities contained in the crude raw material and remove the impurities, obtaining a mineral acid salt of magnesium; adding an alkali to the purified solution so that the pH becomes 10 or higher; subjecting the alkali-containing solution to hydrothermal treatment at a temperature of 120° C. or higher to form a magnesium compound consisting of a double salt of magnesium hydroxide and magnesium sulfate; subjecting the magnesium compound to washing with water and dehydration; and subjecting the dehydrated product to heating at a temperature of 1,000° C. or higher.
With respect to the high-purity magnesium oxide, for example, patent document 2 (Japanese Unexamined Patent Publication No. 2004-084017) discloses magnesium oxide powder used as a raw material for producing a magnesium oxide deposition material for use in the formation of a protective film for a dielectric layer of AC type plasma display panel. The characteristic feature of this magnesium oxide powder resides in that the magnesium oxide powder has an MgO purity of higher than 99.98% by mass and a specific surface area in the range of 5 to 10 m2/g, and the primary particles of the powder have a cubic shape. Further, this patent document discloses that the magnesium oxide powder is produced by a gas phase oxidation reaction method in which high-purity metal magnesium vapor and oxygen are reacted with each other in a gas phase.
This magnesium oxide powder is produced by a gas phase oxidation method in which metal magnesium is directly oxidized, and such a method for producing high-purity magnesium oxide by the gas phase method requires cumbersome facilities for production and complicated operations for reaction. Therefore, a purity of 99.99% by mass or higher is difficult to obtain, and the production cost is considered to be increased, and thus magnesium oxide having a satisfactorily high purity has not yet been obtained.
Patent document 3 (Japanese Unexamined Patent Publication No. 2007-91525) discloses magnesium oxide powder which has a small impurity content and can be easily handled and hence has excellent workability, and which is used as a raw material for forming a protective film for a dielectric layer of color plasma display panel, wherein the magnesium oxide powder contains a primary particle of crystalline magnesium oxide and a secondary particle that is an aggregate of a plurality of the primary particles, and wherein the magnesium oxide powder has an average particle size of 0.5 to 10 μm and a BET specific surface area of 0.1 to 3 m2/g. This patent document discloses that the magnesium oxide powder is produced by firing magnesium oxalate having a predetermined average particle size at a predetermined temperature.
In the production of MgO particles, a method of firing magnesium hydroxide can be used. In this method for producing MgO particles, the magnesium hydroxide as a raw material needs to be high purity. Conventionally, magnesium oxide having a relatively high purity has been produced from magnesium hydroxide, however, there has not yet been obtained magnesium hydroxide having such high purity that the amount of each of the metal impurities contained is 10 ppm by mass or less.
As magnesium hydroxide having a higher purity, for example, patent document 4 (Japanese Unexamined Patent Publication No. 2001-302232) discloses high-purity magnesium hydroxide having a purity of 99.98% by mass or higher in terms of MgO, which is obtained by hydrating MgO fine particles consisting of high-purity single crystals of independent primary fine particles. Further, patent document 5 (Japanese Unexamined Patent Publication No. 2002-255544) discloses that a fired MgO product having a controlled content of a specific inorganic compound is hydrated in the presence of a water-soluble magnesium salt under specific conditions to obtain magnesium hydroxide having a purity of 99.5% by mass or higher.
However, the high-purity magnesium hydroxide disclosed in each of the above prior art documents has a purity as low as less than 99.99% by mass, and has not achieved the purity level of 99.99% by mass or higher, which meets the requirements for the magnesium hydroxide used as a raw material for fluorescent substances and others.
On the other hand, with respect to the magnesium oxide usable in the applications of various additives, electronic materials, raw materials for fluorescent substance, catalyst materials, raw materials for crystalline magnesium oxide layer for PDP and others, the magnesium oxide is required to exhibit excellent dispersibility when the particles of magnesium oxide are dispersed in, e.g., water. For achieving excellent dispersibility, techniques for obtaining an aggregate of particles by granulation using spray drying are disclosed. For example, patent document 6 (Japanese Unexamined Patent Publication No. Hei 08-067505) discloses an inorganic oxide particle (a mother particle) that is an aggregate of inorganic oxide fine particles (child particles), which are produced by spray drying of a slurry comprising an inorganic oxide that is dispersed fine particles (child particles) having an average particle size and particle size distribution in a specific range, for obtaining particles having an average particle size of 0.1 to 1.0 μm and having excellent dispersibility in water. Patent document 7 (Japanese Unexamined Patent Publication No. 2001-327849) discloses granules consisting of dispersible, finely divided solid having a primary particle size of less than 10 μm, wherein the each granule have a uniform density distribution and a substantially spherical shape, and wherein the granules are completely redispersible under dispersion conditions used for the starting solid. However, each of the above prior art documents has no descriptions about studies of the production of high-purity magnesium oxide aggregate.
Generally, magnesium oxide fine particles are prepared by grinding magnesium oxide, which is obtained by firing a precursor, by a physical method using, e.g., a mill or a grinder, and supplied in such a ground state. In the conventional high-purity magnesium oxide, magnesium oxide having purity as high as 99.99% cannot be supplied because of impurities mixed into the magnesium oxide at each step of the production. In addition, ground magnesium oxide by using, e.g., a mill or a grinder is likely to suffer agglomeration, and cannot achieve satisfactory fluidity and dispersibility.
Moreover, there is a problem that, after high-purity magnesium oxide was produced, impurities are mixed into the produced high-purity magnesium oxide powder to lower the purity. It is known that impurities are mixed into the magnesium oxide powder in a large amount especially in the grinding step for the magnesium oxide powder. Therefore, after the high-purity magnesium oxide powder was produced, it is necessary that the purity is kept high.    Patent document 1: Japanese Unexamined Patent Publication No. Sho 61-209911    Patent document 2: Japanese Unexamined Patent Publication No. 2004-084017    Patent document 3: Japanese Unexamined Patent Publication No. 2007-91525    Patent document 4: Japanese Unexamined Patent Publication No. 2001-302232    Patent document 5: Japanese Unexamined Patent Publication No. 2002-255544    Patent document 6: Japanese Unexamined Patent Publication No. Hei 08-067505    Patent document 7: Japanese Unexamined Patent Publication No. 2001-327849